3D Surface Shape Research Articles

High-speed stereo-digital image correlation using a single color high-speed camera.

A simple and practical high-speed stereo-digital image correlation (stereo-DIC) technique using a single off-the-shelf high-speed color CMOS camera is described in this work. By using the high-speed color CMOS camera and suitable optical filters, the recorded color images can be directly separated into red and blue channel sub-images with negligible color cross-talk between sub-channel images, which offers evident efficiency and accuracy advantages over the existing technique we proposed recently [Opt. Lasers Eng.95, 17 (2017)OLENDN0143-816610.1016/j.optlaseng.2017.03.009]. These separated sub-channel images can then be processed by regular stereo-DIC to retrieve the desired kinematic fields on the test object surface. The accuracy and precision of the established high-speed stereo-DIC system were characterized by measuring the displacements of a stationary object, and the results show good agreement with theoretical predications. To show the broad utility and practicality of the proposed method, three typical experiments, involving (i) transient displacement and velocity measurement of a rotating fan; (ii) full-field vibration measurement of a rectangular aluminum panel; and (iii) transient 3D surface shape, displacement, and strain fields measurement of a balloon during the whole explosion procedure, were carried out. The results show that, by using a proper high-speed color camera, high-speed 3D shape, displacement and deformation measurements can be realized in a cost-effective and easy-to-implement manner. The proposed technique demonstrates great potential in impact engineering, explosion, and vibration tests.

An Online Active Balancing Method Using Magnetorheological Effect of Magnetic Fluid

The improvement of machining efficiency and precision puts forward new requirements for the balancing performance of machine tool spindle. Work piece quality can be effectively improved by implementing the active balance on the spindle. In this paper, a new active balancing method using magnetorheological (MR) effect of magnetic fluid is proposed. The mechanism of forming compensation mass by changing the distribution of magnetic fluid under local magnetic field is expounded. Experiments are carried out to verify the feasibility of the proposed method. Profile lines of magnetic fluid surface shape at different positions are measured with linear laser projection measurement method in experiments. Three-dimensional (3D) surface shape of the magnetic fluid is reconstructed by the synthesis of the measured profile lines. Experiments demonstrate that mass center of the magnetic fluid increases with the strength of magnetic field. Thus, the feasibility of the proposed method is verified experimentally. In order to weaken the vibration of machine tool spindle using this method, a balancing device is designed, which includes magnetic fluid chambers and three conjugated C-type electromagnets arranged at 120 deg intervals. For each electromagnet, the relationship among compensation mass (the corresponding balancing mass), excitation current, and rotation speed is established. Also, the performance of the balancing device is further proved in experiments conducted on the experimental platform. The imbalance vibration amplitude of the test spindle decreased by an average of 87.9% indicates that the proposed active balancing method in this paper is promising.

An Improved Method for Restoring the Shape of 3D Point Cloud Surfaces

Building 3D objects or reconstructing their surfaces from 3D point cloud data are researched activities in the field of geometric modeling and computer graphics. In the recent years, they are also studied and used in some fields such as: graph models and simulation; image processing or restoration of digital heritages. This article presents an improved method for restoring the shape of 3D point cloud surfaces. The method is a combination of creating a Bezier surface patch and computing tangent plane of 3D points to fill holes on a surface of 3D point clouds. This method is described as follows: at first, a boundary for each hole on the surface is identified. The holes are then filled by computing Bezier curves of surface patches to find missing points. After that, the holes are refined based on two steps (rough and elaborate) to adjust the inserted points and preserve the local curvature of the holes. The contribution of the proposed method has been shown in processing time and the novelty of combined computation in this method has preserved the initial shape of the surface

A New Oren–Nayar Shape-from-Shading Approach for 3D Reconstruction Using High-Order Godunov-Based Scheme

3D shape reconstruction from images has been an important topic in the field of robot vision. Shape-From-Shading (SFS) is a classical method for determining the shape of a 3D surface from a one intensity image. The Lambertian reflectance is a fundamental assumption in conventional SFS approaches. Unfortunately, when applied to characterize the reflection attribute of the diffuse reflection, the Lambertian model is tested to be inexact. In this paper, we present a new SFS approach for 3D reconstruction of diffuse surfaces whose reflection attribute is approximated by the Oren–Nayar reflection model. The partial differential Image Irradiance Equation (IIR) is set up with this model under a single distant point light source and an orthographic camera projection whose direction coincides with the light source. Then, the IIR is converted into an eikonal equation by solving a quadratic equation that includes the 3D surface shape. The viscosity solution of the resulting eikonal equation is approximated by using the high-order Godunov-based scheme that is accelerated by means of an alternating sweeping strategy. We conduct the experiments on synthetic and real-world images, and the experimental results illustrate the effectiveness of the presented approach.

Full-Field Surface 3D Shape and Displacement Measurements Using an Unfocused Plenoptic Camera

Full-field surface 3D shape and displacement measurements using a single commercial unfocused plenoptic camera (Lytro Illum) are reported in this work. Before measurements, the unfocused plenoptic camera is calibrated with two consecutive steps, including lateral calibration and depth calibration. Each raw image of a checkerboard pattern recorded by Lytro Illum is first extracted to an array of sub-aperture images (SAIs), and the center sub-aperture images (CSAIs) at diverse poses are used for lateral calibration to determine intrinsic and extrinsic parameters. The parallax maps between the CSAI and the remaining SAIs at each pose are then determined for depth parameters estimation using depth calibration. Furthermore, a newly developed physical-based depth distortion model is established to correct the serious distortion of the depth field. To realize shape and deformation measurements, the raw images of a test sample with speckle patterns premade on its surface are captured by Lytro Illum and extracted to arrays of SAIs. The parallax maps between the CSAI and the target SAIs are obtained using subset-based digital image correlation. Based on the pre-computed intrinsic and depth parameters and the disparity map, the full-field surface 3D shape and displacement of a test object are finally determined. The effectiveness and accuracy of the proposed approach are evaluated by a set of experiments involving the shape reconstruction of a cylinder, in-plane and out-of-plane displacement measurements of a flat plate and 3D full-field displacement measurements of a cantilever beam. The preliminary results indicate that the proposed method is expected to become a novel approach for full-field surface 3D shape and displacement measurements.

Optimal Color Multiplexing for Low-Cost Structured Light 3D Capture System with Two Projectors

3D shape reconstruction is one of the most important topics in computer vision and the foundation for a wide field of application. Among various technologies, structured light is one of the most reliable techniques. However, given the field of view of projectors and cameras available in the market, the working distance needed for projectors is typically larger than that for cameras. To reduce the working distance of the projectors while covering the whole working platform, two projectors with their field of view overlapping are used to cover the working area which holds objects to be scanned. We present a spectral analysis based model for the projector-camera system, in order to find the most distinguishable colors for two projectors, and best separate the projected patterns from two projectors. The optimal values of the two colors are determined by the pattern search method in the presence of noise, which is modeled as multivariate Gaussian noise, and characterized for different input colors. The camera sensors' responses to the projector are measured after linearization with gray balance curves. After being properly calibrated, based on one image shot of the object with binary M-array patterns projected on it, the system can reconstruct a 3D shape of the object surface.

3D numerical simulation of free surface shape during the crystal growth of floating zone (FZ) silicon

In FZ growth processes, the stability of the free surface is important in the production of single crystal silicon with high quality. To investigate the shape of the free surface in the FZ silicon crystal growth, a 3D numerical model that included gas and liquid phases was developed. In this present study, 3D Young-Laplacian equations have been solved using the Volume of Fluid (VOF) Model. Using this new model, we predicted the 3D shape of the free surface in FZ silicon crystal growth. The effect of magnetic pressure on shape of free surface has been considered. In particular, the free surface of the eccentric growth model, which could not be previously solved using the 2D Young-Laplacian equations, was solved using the VOF model. The calculation results are validated by the experimental results.

Dual-layer orthogonal fiber Bragg grating mesh based soft sensor for 3-dimensional shape sensing.

A soft shape sensor for 3-dimensional object shape measurement is demonstrated. The proposed sensor is based on dual-layer fiber Bragg grating arrays with an orthogonal mesh structure, which enables multi-point bi-directional shape sensing. The 3D shape reconstruction is based on a bi-directional curvature measurement at each sensing point, which is achieved through measuring the direction and amount of wavelength shift of each off-center embedded FBG. The conversion coefficient between the wavelength shift and bending curvatures is acquired and is used to convert the change in FBG to the corresponding bending curvatures and bending direction. The measurement error on the bending radius of each sensing point is about 2.7%. A 3D shape of the object surface is reconstructed with the help of a curve fitting method based on the curvature information across the whole FBG mesh. This design successfully achieved visualized 3D shape sensing, which has great practical value in soft robotics and biomedical applications.

Model-based inverse estimation for active contraction stresses of tongue muscles using 3D surface shape in speech production

This paper presents a novel inverse estimation approach for the active contraction stresses of tongue muscles during speech. The proposed method is based on variational data assimilation using a mechanical tongue model and 3D tongue surface shapes for speech production. The mechanical tongue model considers nonlinear hyperelasticity, finite deformation, actual geometry from computed tomography (CT) images, and anisotropic active contraction by muscle fibers, the orientations of which are ideally determined using anatomical drawings. The tongue deformation is obtained by solving a stationary force-equilibrium equation using a finite element method. An inverse problem is established to find the combination of muscle contraction stresses that minimizes the Euclidean distance of the tongue surfaces between the mechanical analysis and CT results of speech production, where a signed-distance function represents the tongue surface. Our approach is validated through an ideal numerical example and extended to the real-world case of two Japanese vowels, /ʉ/ and /ɯ/. The results capture the target shape completely and provide an excellent estimation of the active contraction stresses in the ideal case, and exhibit similar tendencies as in previous observations and simulations for the actual vowel cases. The present approach can reveal the relative relationship among the muscle contraction stresses in similar utterances with different tongue shapes, and enables the investigation of the coordination of tongue muscles during speech using only the deformed tongue shape obtained from medical images. This will enhance our understanding of speech motor control.

O-CNN

We present O-CNN , an Octree-based Convolutional Neural Network (CNN) for 3D shape analysis. Built upon the octree representation of 3D shapes, our method takes the average normal vectors of a 3D model sampled in the finest leaf octants as input and performs 3D CNN operations on the octants occupied by the 3D shape surface. We design a novel octree data structure to efficiently store the octant information and CNN features into the graphics memory and execute the entire O-CNN training and evaluation on the GPU. O-CNN supports various CNN structures and works for 3D shapes in different representations. By restraining the computations on the octants occupied by 3D surfaces, the memory and computational costs of the O-CNN grow quadratically as the depth of the octree increases, which makes the 3D CNN feasible for high-resolution 3D models. We compare the performance of the O-CNN with other existing 3D CNN solutions and demonstrate the efficiency and efficacy of O-CNN in three shape analysis tasks, including object classification, shape retrieval, and shape segmentation.

A fast pixel matching method based on phase feature extraction in online phase-measuring profilometry

A fast pixel matching method based on phase feature extraction in online phase-measuring profilometry is proposed, which is used to solve the problem where the object motion is along a straight line. The movement of the object results in the displacement of images in the captured deformed patterns, so pixel matching is carried out to modify the positions of images, which not only realizes the point-to-point correspondence of the object, but extracts the equivalent phase-shifting deformed patterns. So, the 3D surface shape of the object moving on the pipeline can be reconstructed successfully with an equivalent phase-shifting algorithm. The equiphase surface phase feature extracted from the phase distributions of the object obtained by Fourier transform profilometry is used to assist pixel matching in this paper. Normalization and further binarization of the phase feature extraction improve pixel matching’s accuracy and speed. The simulation and experiments verify the feasibility and validity of the proposed method.

Specular reflection compensation in homography fringe projection profilometry

Fringe projection profilometry (FPP) is widely used in the industrial 3D measurement. When measuring an object using FPP, inclined projection will cause problems of periodic inconsistency and light-intensity inhomogeneity to fringe patterns. This paper presents an improved homography fringe generation method to correct the fringes. In this paper, a homography transform is used not only for the coordinate transformation between the projection plane and the reference plane, but also for the modulation of the period and light intensity of the fringe. Based on this method, the captured fringe patterns have good periodicity and homogenized intensity, meanwhile, the impact of high reflection is evidently weakened in the recovery of 3D surface shape with specular component, thus effectively reducing the phase errors. Experiments show that the 3D rail shape is well recovered and the error is apparently decreased based on the proposed method.

3D surface profilometry based on 2D S-Transform method with optimized window

S-Transform method can be regarded as an extension to the ideas of the Windowed Fourier Transform (WFT) method and the Wavelet Transform (WT) method, which gives consideration to both local frequency concept of WFT and the multi-resolution strategy of WT. It has been introduced in optical 3D shape measurement recently. However, the S-Transform method suffers from poor energy concentration when some kind of signal is treated with. In order to enhance energy concentration to provide more accurate estimation of the instantaneous frequency, a modification to the existing 2D S-Transform is proposed in this paper. Two additional parameters are introduced to respectively optimize the window width in x and y direction. The paper gives the expression of 2D S-Transform with optimized window and its performance in 3D surface reconstruction. The application of proposed method is demonstrated on both simulation and experiment by comparing with traditional 2D S-Transform method. The improved performance of optimized 2D S-Transform for 3D surface shape measurement shows its feasibility and veracity.

3D precision measurements of meter sized surfaces using low cost illumination and camera techniques

Using dedicated stereo camera systems and structured light is a well-known method for measuring the 3D shape of large surfaces. However the problem is not trivial when high accuracy, in the range of few tens of microns, is needed. Many error sources need to be handled carefully in order to obtain high quality results. In this study, we present a measurement method based on low-cost camera and illumination solutions combined with high-precision image analysis and a new approach in camera calibration and 3D reconstruction. The setup consists of two ordinary digital cameras and a Gobo projector as a structured light source. A matrix of dots is projected onto the target area. The two cameras capture the images of the projected pattern on the object. The images are processed by advanced subpixel resolution algorithms prior to the application of the 3D reconstruction technique. The strength of the method lays in a different approach for calibration, 3D reconstruction, and high-precision image analysis algorithms. Using a 10 mm pitch pattern of the light dots, the method is capable of reconstructing the 3D shape of surfaces. The precision (1σ repeatability) in the measurements is <10 µm over a volume of 60 × 50 × 10 cm3 at a hardware cost of ~2% of available advanced measurement techniques. The expanded uncertainty (95% confidence level) is estimated to be 83 µm, with the largest uncertainty contribution coming from the absolute length of the metal ruler used as reference.

A dual-frequency fringe projection three-dimensional shape measurement system using a DLP 3D projector

A dual-frequency fringe projection system for three-dimensional (3D) surface shape measurement is proposed in this paper. The system includes two cameras, a DLP 3D projector, and a liquid crystal (LC) shutter glasses. The phase information related to the object height is obtained from the dual-frequency temporal method with 3-step phase-shifting algorithm. By using the DLP 3D projector and LC shutter glasses, 3-step phase-shifting high-frequency and low-frequency fringe patterns are captured only 3 times by the two cameras synchronously. The technique of image registration is applied to low-frequency fringe patterns to guarantee the accuracy of low-frequency phase for high-frequency phase unwrapping. Using the equi-phase coordinate method based on two reference planes, the phase-to-height conversion and non-sinusoidal errors reduction are carried out in one go without any extra operation or measurement time. Experimental results demonstrate that the proposed method effectively improves the measuring speed, and it is valid for measuring surface shapes with multi-steps or discontinuities.

A Bayesian approach to simultaneously recover camera pose and non-rigid shape from monocular images

In this paper we bring the tools of the Simultaneous Localization and Map Building (SLAM) problem from a rigid to a deformable domain and use them to simultaneously recover the 3D shape of non-rigid surfaces and the sequence of poses of a moving camera. Under the assumption that the surface shape may be represented as a weighted sum of deformation modes, we show that the problem of estimating the modal weights along with the camera poses, can be probabilistically formulated as a maximum a posteriori estimate and solved using an iterative least squares optimization. In addition, the probabilistic formulation we propose is very general and allows introducing different constraints without requiring any extra complexity. As a proof of concept, we show that local inextensibility constraints that prevent the surface from stretching can be easily integrated.An extensive evaluation on synthetic and real data, demonstrates that our method has several advantages over current non-rigid shape from motion approaches. In particular, we show that our solution is robust to large amounts of noise and outliers and that it does not need to track points over the whole sequence nor to use an initialization close from the ground truth.

Experimental and Numerical Study of Pile-Stabilized Slopes Under Surface Load Conditions

This paper is a comprehensive analysis on various experimental and numerical studies on slopes. Small-scale physical modeling of slopes under surcharge loads was performed on loose sand slopes. Digital images were captured during the incremental loading from the model’s side. Particle image velocimetry and 3D numerical model were applied to investigate the slope under surcharge loading; several important factors that control pile performance are investigated. Safety factors and location of critical failure surfaces of various reinforced and unreinforced slopes are obtained and compared. The results shows, the 3D failure surface shape in front of the piles is triangular for stabilized homogenous slopes, which is different from its conical shape in laterally loaded piles. This paper also analyzes the effects of soft bound interlayer, soil properties, pile spacing, pile position, and surcharge distance on the safety factor and slip surface shape of the stabilized and non-stabilized slopes. The results indicate that the distance of soft soil layer from ground surface and its horizontal direction angle strongly influence the optimum location of the piles. The numerical modeling results show that increasing the distance between the piles prevents the arching phenomenon from developing, and instead, increases the failure volume in the slope.

A measurement system applicable for landslide experiments in the field

Observation of gravity erosion in the field with strong sunshine and wind poses a challenge. Here, a novel topography meter together with a movable tent addresses the challenge. With the topography meter, a 3D geometric shape of the target surface can be digitally reconstructed. Before the commencement of a test, the laser generator position and the camera sightline should be adjusted with a sight calibrator. Typically, the topography meter can measure the gravity erosion on the slope with a gradient of 30°-70°. Two methods can be used to obtain a relatively clear video, despite the extreme steepness of the slopes. One method is to rotate the laser source away from the slope to ensure that the camera sightline remains perpendicular to the laser plane. Another way is to move the camera farther away from the slope in which the measured volume of the slope needs to be corrected; this method will reduce distortion of the image. In addition, installation of tent poles with concrete columns helps to surmount the altitude difference on steep slopes. Results observed by the topography meter in real landslide experiments are rational and reliable.

Fourier transform profilometry of a single-field fringe for dynamic objects using an interlaced scanning camera

Fourier transform profilometry (FTP) of a single-field fringe is proposed using an interlaced scanning camera in dynamic object's surface shape measurement. The deformed fringe patterns of a measured dynamic object are grabbed by an interlaced scanning camera, every of which is divided into two single-field fringes, each of which is not necessarily deinterlaced, and directly reconstructs a 3D surface shape by using FTP. A conclusion is inferred theoretically that two reconstructed object's shapes are identical using a single-field fringe and its corresponding frame fringe. The simulation and experiment results demonstrate the feasibility and effectiveness of the proposed method. The method not only keeps the advantages of traditional FTP, but also doubles the measured system's time resolution.

Study on 3D shape and deformation measurement of a radiator fan based on high-speed binary defocused fringe projection

The binary defocused fringe can overcome the nonlinear effect of an ordinary commercial digital projector and generate a good sinusoidal light field. In this paper, the binary fringe encoding method was chosen to complete the high-speed fringe projection using a DLP Lightcrafter 4500 model in dynamic Fourier transform profilometry, and then a camera was synchronously triggered to record the deformation fringes on the surface of a high-speed rotating radiator fan, whose time-changing 3D surface shape was reconstructed with Fourier fringe analysis. Further deformation analysis of the measured radiator fan blade and rotating shaft is helpful to improve the performance of the radiator. The experimental results on a tested radiator fan with the rotation speed of 4800rpm identified the validity of the presented setup, and this method can effectively expand the application of dynamic 3D shape measurement.